Chapter 3

# Chapter 3 - Chapter 3 Stoichiometry Chemical Calculations...

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Chapter 3: Stoichiometry: Chemical Calculations 1 Balancing Chemical Equations 2H 2 + O 2 → 2H 2 O 4 H atoms + 2 O atoms 4 H atoms + 2 O atoms This equation is balanced – same number of atoms for each element on both sides of equation. Remember, in chemical reactions atoms are neither created nor destroyed; so they all must be accounted for. Equal # atoms on both sides of equation. Formula Unit – one unit (atom, ion, molecule) corresponding to given formula 1 formula unit silicon = 1 Si atom 1 formula unit water = 1 H 2 O molecule 1 formula unit MgCl 2 = 1 Mg 2+ ion and 2 Cl - ions Use this concept to balance equations. General Guidelines to Balance Equations 1) Write unbalanced equation using correct formulas for compounds. correct: Al + Cl 2 → AlCl 3 incorrect: Al + Cl 2 → AlCl 2 not formula for aluminum chloride never balance equations by manipulating formulas 2) Balance equation using coefficients in front of formulas. Al + Cl 2 → AlCl 3 Al balanced, but . .. 2Al + Cl 2 → 2AlCl 3 gives even # Cls 2Al + 3Cl 2 → 2AlCl 3 balance Cls This is somewhat creative process. You’ll get feel for how to do it. Some Guidelines: DON’T balance element in 2 or more compounds on same side first. If don’t know where to start put or imagine a “1” coefficient in front of the most complicated compound and go from there. 3) Make sure coefficients are least common multiples 4Al + 6Cl 2 → 4AlCl 3 needs to become 2Al + 3Cl 2 → 2AlCl 3 4) Check to make sure same # atoms of each element on both sides of equation. Example: Combustion of butane C 4 H 10 + O 2 → CO 2 + H 2 O Step 1: These are correct formulas so they do not change. Step 2: What element should NOT be balanced first? Oxygen C 4 H 10 + O 2 → CO 2 + H 2 O C 4 H 10 + O 2 → 4CO 2 + H 2 O C balanced 4 C 4 C C 4 H 10 + O 2 → 4CO 2 + 5H 2 OH balanced

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Chapter 3: Stoichiometry: Chemical Calculations 2 10 H 10 H now have 8 + 5 = 13 O on this side C 4 H 10 + 13 2 O 2 → 4CO 2 + 5H 2 O 13 O 13 O Step 3: Need to multiply × 2 C 4 H 10 + 13 2 O 2 → 4CO 2 + 5H 2 O × 2 2C 4 H 10 + 13O 2 → 8CO 2 + 10H 2 O balanced Step 4: 8 C 20 H 26 O 8 C 20 H 26 O In lab we need a way to go beyond looking at coefficients as representing individual atoms, molecules, ions. Avogadro’s Number and the Mole Suppose you want to do the following reaction: Na + Br 2 → NaBr 2Na + Br 2 → 2NaBr balanced You couldn’t measure out 2 atoms Na, 1 molecule Br 2 Need another way to measure this. Mole
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Chapter 3 - Chapter 3 Stoichiometry Chemical Calculations...

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